Shield Door
From Halopedia, the Halo wiki
One of the Shield Doors on Snowbound.
Shield Doors are Covenant and Forerunner made "doors" that are able to allow people and machines through while repelling plasma and gunfire. They are similar in design to energy shields that the Jackals wear.
There are several on the in past Halo games and they appear in Halo 3 on multiplayer map Snowbound.
Now they are also featured as forge items in the new Heroic Map Pack, and they can be placed anywhere.
If one stands exactly in the sheild door's energy field, doing a melee or shooting in any direction will deflect off the sheild. However, the player is still vulnerable from attacks from either side. This also works with the Bubble Shield.
